Features of structures силикатов

In a basis of structure силикатов lays тетраэдр [SiO4] formed(educated) at sp3- гибридизации валентных электронов. The ions of oxygen тетраэдрически surround an ion of silicon and are on distance 1,62·10-10 м from him(it). Practically in all connections the silicon saves coordination number 4, as a parity(ratio) rк: rа = 0,39. At comparison of an ion [SiO4] 2- with others тетраэдрическими by ions [РO4] 3-, [SiO4] 2- it is visible, that he is characterized by the least force of communication(connection). On Полингу, communication(connection) Si - O on 50 % ионная, the energy of communication(connection) makes ~ 470 кДж/моль. Therefore силикаты on the properties are closer to оксидам, than to acids. Тетраэдры [SiO4] 4- сочленяются with each other by tops through atoms of oxygen. General(common) edges and sides тетраэдры have no, therefore such structures are малоустойчивыми. As тетраэдр has four tops, two can be general(common) one,, three and all four tops. It defines(determines) all variety of structures and various attitude(relation) Si: O in силикатах. Various under the form of a combination mutually connected тетраэдров [SiO4] 4- are called кремнекислородными as motives.

Кремнекислородные motives can be of the final and infinite sizes. They образую of a ring, chain, tapes, layers, skeletons. Characteristic feature of a structure силикатов is the opportunity of replacement in crystal structure of ions of silicon a little bit large on the size by ions of aluminium. The attitude(relation) rк: rа for aluminium makes 0,415, i.e. is boundary. Therefore aluminium can have coordination number 4 and 6. The aluminium with coordination number 4 can enter in кремнекислородный motive. The aluminium with coordination number 6 plays a role катиона.

Силикаты, in which the aluminium replaces silicon, are called алюмосиликатами. Силикаты, in which the aluminium does not enter in кремнекислородный motive, are called силикатами of aluminium.

Sometimes in силикатах contain additional анионы - more often О2-, he, F-, and also neutral molecules of water. Most often meeting in силикатах катионами are Мg2 +, Fe2 +, Al3 +, Ca2 +, K +, Na +, Mn2 +, Zr4 +, Ti4 + Small on the size катионы Мg2 +, Fe2 +, Al3 + usually have coordination number 6. Such катионы as Ca2 +, Zr4 +, Ti4 +, can have coordination numbers 6 and 8. Large однозарядные катионы K +, Na + have coordination numbers 8 and even 12.

In силикатах is extremely advanced изовалентный and гетеровалентный изоморфизм.

In a basis of structural classification силикатов and алюмосиликатов the way сочленения тетраэдров is fixed [SiO4] 4- in a crystal lattice.

On structural classification offered Брегом and Ф.Махачки all силикаты is possible to divide(share) on two large groups: силикаты with кремнекислородными by motives of the final sizes and кремнекислородными by motives infinite in one or several directions of the sizes.

 

Structure силикатов

 

Силикаты with кремнекислородными Силикаты with кремнекислородными

By motives of the final sizes by motives of the final sizes

 

силикаты with изоли- силикаты with Силикаты with Силикаты with Силикаты with

рованными оди- by groups from one-dimensional двухмерными three-dimensional

Night тетра- тетраэдров by chains by layers from continuous

эдрами [SiO4] 4- [SiO4] 4- or tapes тетраэдров skeleton from

( Island final тетраэдров [SiO4] 4- тетраэдров

струтуры) sizes [SiO4] 4- (це- (layered [SiO4] 4-

почечные and structure) (frame

Tape structures)

Structures)

 

Structures силикатов with кремнекислородными by motives of the final sizes.

1. Structure силикатов with isolated single тетраэдрами [SiO4] 4- (island structures) In these structures тетраэдры [SiO4] 4- are not connected directly to others similar тетраэдрами through atoms of oxygen, and incorporate through катионы of metals which are included in structure силикатов, i.e. кремнекислородный the motive in similar силикатах has structure [SiO4] 4-. Isolated тетраэдры [SiO4] 4- name ортогруппами, and силикаты, containing ортогруппы, - ортосиликатами. To them carry силикаты, in which the attitude(relation) of number of atoms of oxygen to atoms of silicon equally or is more 4. The minerals of this group represent силикаты, in which тетраэдры [SiO4] 4- do not incorporate among themselves by tops. Each ion of oxygen belongs to one тетраэдру. Groups [SiO4] 4- incorporate among themselves катионами. Into structure these силикатов enter small and average двухзарядные катионы Мg2 +, Fe2 +, Ca2 +, Mn2 +. Such катионы, as K +, Na +,, in them does not meet almost, and Al3 + never replaces Si4 +. The main representatives island силикатов are the minerals of groups оливинов (форстерит Mg2 [SiO4], фаялит Fe2 [SiO4]), силлиманита, белит Ca2 [SiO4], mineral of group гранатов гроссуляр, монтичелит CaMg [SiO4], and also циркон.

2. Structure силикатов with groups from тетраэдров [SiO4] 4- final sizes. Into this group enter силикаты, which structure contains groups of the final sizes from тетраэдров [SiO4] 4-, connected among themselves through general(common) (мостиковые) atoms of oxygen. The form and the sizes of oxygen motives in силикатах of this group can be various. A radical [Si2O7] 6- (диортогруппа) consists of two тетраэдров [SiO4] 4-, connected by general(common) top, i.e. one atom of oxygen is general(common) for two тетраэдров. Силикаты, containing such radicals, are called диортосиликатами or пиросиликатами. The representatives similar силикатов are the minerals: окерманит Ca2Mg [Si2O7], ранкинит Ca3 [Si2O7], гидрат трехкальциевого силиката Ca6 [Si2O7] (HE) 6, etc.

Other type кремнекислородных of motives of the final sizes arises in the event that a little тетраэдров incorporate in a ring, forming ring кремнекислородные radicals. Thus, if in everyone тетраэдре обобществляются two аниона of oxygen, can be formed unary three, four and шестичленные rings containing accordingly 3, 4 and 6 кремнекислородных тетраэдров, each of which has two general(common) atoms of oxygen with next тетраэдрами. The radical appropriate трехчленному to a ring, has structure [Si3O9] 6-, четырехчленному - [Si4O12] 8- and шестичленному - [Si6O8] 12-. Except for the specified types of radicals in structures силикатов there are also radicals [Si5O15] 10- - five сочлененных тетраэдров and [Si12O30] 12- - dual ("two-storeyed") шестичленное a ring (three general(common) atoms of oxygen in everyone тетраэдре). By an example силиката with кремнекислородным by motive [Si3O9] 6- is бентонит BaTi [Si3O9]; шестичленные of a ring with a radical [Si6O18] 12- contain in structure of such minerals, as берилл BeAl2 [Si6O18], кордиерит Mg2Al3 [AlSi5O18], in кремнекислородном which radical, as it is visible from the structural formula, one atom of silicon is replaced on aluminium.

Structure силикатов with кремнекислородными by motives of the infinite sizes.

1. Structures with one-dimensional (infinite in one measurement) chains or tapes from тетраэдров [SiO4] 4- chained and tape structures. Тетраэдры [SiO4] 4-, incorporating among themselves by tops, can form unary chains, infinite in one direction. Everyone тетраэдр in a chain will have two general(common) atoms of oxygen with two next тетраэдрами and two free valencies, through which катионы of metals will connect chains in a lattice. The number тетраэдров in a chain beyond all bounds in the sense that is defined(determined) only by size of the given crystal. The structure кремнекислородного of motive in structures with infinite radicals is defined(determined) by structure of the period of repeatability (identity) of this motive made in square brackets, i.e. that structural element, which infinite recurrence in this or that direction is formed кремнекислородный motive by [SiO3] 2-? (one mark of infinity specifies infinity of a chain in one direction).

Other motive of this structural group силикатов is formed, if two infinite chains incorporate through tops тетраэдров together, forming dual infinite in one direction of a tape or belt(zone) (tape structures). Thus the motives as tapes can be formed various on structure and configuration: the dual chain with четырехчленными by rings having a radical of structure [Si2O5] 2-?, dual chain with шестичленными by rings with a radical [Si4O11] 6-?, dual chain with восьмичленными by rings with a radical [Si4O11] 10-?. The minerals with chained and tape кремнекислородными by radicals make the large group. Chained кремнекислородные motives of structure [SiO3] 2-?, have, for example, метасиликаты (силикаты with the attitude(relation) О: Si =3). пироксенов, in particular, concern to them minerals of group: энстатит Mg [SiO3], диопсид Са Mg [SiO3] 2 etc. Tape кремнекислородные radicals have the minerals of group амфиболов - connections, as a rule, of complex(difficult) structure similar to structure пироксенов, but as against them, containing group he. To амфиболам such minerals, as horn обманка, тремолит and пр concern.

2. Structures with двухмерными by layers from тетраэдров [SiO4] 4 - layered structures. If everyone тетраэдр [SiO4] 4- incorporates by three tops with next тетраэдрами, the layers (sheets or grids) can be formed infinite in two measurements.

The structure and character of radicals in layered силикатах also can be various. The most simple and widespread type of a layer - кремнекислородный the layer гексагонального of a type representing sheets with шестичленными by rings, has a radical [Si2O5] 2-??. Such minerals concern to this structural type, as каолинит Al2 [Si2O5] (HE) 4, пластинчатые minerals such as micas, for example мусковит КAl2 [AlSi3O10] (HE) 2, тальк Mg3 (HE) [Si2O5] 2 2, and also such minerals of more complex(difficult) structure, as биотит, вермикулит etc.

To the same type of structures concern силикаты, having кремнекислородные layers from four, five and восьмичленными by rings.

3. Structures with a three-dimensional continuous skeleton from тетраэдров [SiO4] 4- - frame structures. In силикатах with frame structure all 4 atoms of oxygen everyone тетраэдра are general(common) with atoms of oxygen of four tops тетраэдра the three-dimensional skeleton is formed infinite in three measurements. As on each atom of silicon thus it is necessary 2 atoms of oxygen, the structure of a radical will be expressed by the formula [SiO2]. Such radical have the various updatings кремнезема SiO2 (quartz, тридимит, кристобалит). The frame structures of updatings SiO2 do not submit to a principle of most dense packing and represent "friable", "ажурные" of structure containing rather large structural emptiness.

In clean(only) кремнеземистом a skeleton of valency of silicon are completely compensated by valencies of oxygen. However if четырехвалентный the silicon in such skeleton изоморфно is replaced, for example, трехвалентным by aluminium, the skeleton gets a negative charge, for which indemnification the introduction in emptiness of structure additional катионов of metal is necessary. The similar structure will consist from тетраэдрических of groups [SiO4] 4- and [Al O4] 5-, and all тетраэдры will be had by(with) general(common) tops. The structure mixed алюмокремникислородного of motive can be expressed thus by the general(common) formula [(Si, Al) O] n-. Frame structures of many widespread natural and artificial minerals such as field шпатов, цеолитов etc. thus are formed

In a basis of structure field шпатов, being one of basic породообразующих of natural minerals and having the large technical meaning(importance) (for example in technology of ceramics), the skeleton from connected among themselves тетраэдрических of groups lays [SiO4] 4- and [Al O4] 5- (изоморфное replacement Si4 + on Al 5 +), and in emptiness of this negatively charged skeleton settle down compensating a charge катионы of alkaline and щелочно-ground metals (To, Na, Ca, Ba)

 

Structure refractory simple and complex(difficult) оксидов, карбидов, боридов and силицидов.

 

To simple concern оксиды, containing with what - or one kind of atom метала, to complex(difficult) - оксиды, into which structure enter of two and more various kinds of atoms of metals. To last, in particular, belong numerous technically important double оксиды such as шпинелей.

Rather widespread(rather distributed) among simple оксидов of metals with the general(common) formula Ме2+О is структуный a type of stone salt (NaCl) .Кубическую a lattice with structure NaCl have MgO, CaO, SrO, BaO, CdO, Vo, MnO, FeO, CoO, NiO.

Feature of a structure of crystal lattices оксидов are that their basis is made dense cubic or гексагональная by(with) packing of ions of oxygen. In similar оксидов with small ионным in radius катиона метала this катион borrows(occupies) all октаэдрические of emptiness in dense packing from анионов of oxygen.

In some simple оксидах with the general(common) formula Ме2+О, кристаллизующихся in in a structural type вюрц ита, for example Pb, BeO, ZnO, катионы метала settle down periodically in half тетраэдрических of emptiness dense гексагональной of packing of atoms of oxygen.

In simple оксидах with the general(common) formula Ме23+О3 often there is a structural type корунда, to which belongs except for? -Al2O3, for example, structure гематита? - Fe2O3, Cr2O3, V2O3 and тд.

Many оксиды with the simple formula Ме4+О2 crystallize in a structural type флюорита CaF2, for example: NiO2, CaO, VO2, ZrO2 (гексагональная structure флюорита) or structural type рутила TiO2, for example: GeO2, TiO2, WO2 etc.

The large group complex(difficult) double оксидов crystallizes in a structural type шпинели. To шпинелям concern crystallized in cubic system double оксиды with the general(common) formula Ме2+О * Ме23+О3,

Where: Ме2 + - Mg, Fe, Mn, Zn, Be, Ni, Co, Cr, Cd, Pb,

Ме3 +- - distinguish алюмошпинели (for example noble шпинель MgO *Al2O3, герценит FeO* Cr2O3), хромошпинели (for example ферохромит FeO* Cr2O3) and т.д

The similarity of structures and parameters of a lattice smaller шпинелей causes one of features - ease of education between them of firm solutions of replacement.

Карбид of silicon (карборунд) - углеродистый the silicon for the first time was received in 1824г. by the Swedish chemist Берцелиусом. Two polymorphic crystal updatings карбида of silicon - cubic (lattice such as diamond а=0,4359нм.) and гексагональная (layered structure with parameters а=в=0,3078 нм are known. And с=0,2518 n нм, where n- number of layers in гексагональной to a cell). In an interval of temperatures 2100-2400о With the cubic updating монотропно passes in гексагональную.

Карбид meets in a nature a kind of fine crystals, sometimes in association smallest crystals of diamond. The crystals карборунда are found out in meteoric craters, diamond трубках, volcanic eruptions.

In industrial conditions карбид of silicon receive in electric furnaces at 2000-2200о With and heating of a mix of quartz with coke or термоантрацитом on reaction requiring(demanding) the large expenses of heat:

SiO2 + 3C SiC + 2CO -Q

Or (at smaller expenses of heat) direct method from silicon and carbon.

Chemically pure(clean) карбид of silicon is colourless, and technical the black colour by impurity Fe, Al, Mg, Zn, Ti, Cr is painted in greenish colour or сине -. Карборунд chemically of racks. He does not react with acids, except for a mix concentrated HF and HNO3.

Расплавленные metals Mg, Mn, Al, Fe, Co, Ni, Cr, Pt, cooperate with карбидом of silicon, whereas Zn, Pb, Cu with it(him) do not react.

Карборундовые materials differ high теплопроводностью, термостойкостью, stability(resistance) to corrosive attack of sour slags, абразивной by stability(resistance) and heat resistance, concerning high electrical conductivity.

Карборунд is used in quality абразивного of a material (powders, circles, grinding and наждачная a paper) at processing metals and alloys, stone, and as for manufacturing high-temperature heaters. The most important areas of application карбида of silicon are connected to his(its) special electrical, including semi-conductor properties.